Cytosolic adaptor proteins containing Immunoreceptor tyrosine-based activation jjiotifs (ITAMs) are long- recognized components of immunoreceptor signaling in cells of the innate and adaptive immune system More recently, ITAM-bearing proteins have also been shown to play a previously unsuspected role in trans mitting activation signals into cells following the binding of adhesive ligands to a select number of cell surface integrins. We have recently discovered that the major platelet integrin, allb^3, enlists the ITAM-bearing transmembrane receptor, FcyRlla, in order to transmit adhesion-initiated, outside-in signals into the cell. How FcyRlla couples functionally to mediate allb;ff3 outside-in signaling, however, is not known, and many of the molecular details underiying the contribution of this newly-recognized component of allb)ff3-mediated signal transmission remain to be uncovered. This proposal, therefore, will address four interrelated Specific Aims focused on integrin/ITAM signaling in platelets. Specific Aim 1 will examine the molecular interactions between allb;ff3, Fc/Rlla, and fibrinogen that might explain their functional coupling, with the goal of clarify ing the topographical relationship of this physiologically-important integrin/ITAM pair, and defining the re quirements for their functional coupling. Aim 2 seeks to test the hypothesis that the focal adhesion kinase, FAK, plays a role in integrin/ITAM coupling, thereby linking what were previously thought to be two separate and independent outside-in signaling pathways. Specific Aim 3 proposes to identify ITAM-bearing proteins that couple functionally to the human platelet integrin a5p^ and to mouse a\\hfi3, with the hopes of better understanding the rules that govern functional coupling between these integrins and ITAM-bearing receptors, and providing new conceptual insights into the overall process of outside-in integrin signaling in platelets and other cells. Finally, Specific Aim 4 will evaluate the importance of allb)ff3/FcKRIIa functional coupling in a number of in vitro and in vivo models of thrombosis and hemostasis. Taken together, we expect that findings made as a result of this investigation will provide an improved understanding of the molecular mechanisms that regulate platelet activation, and lay a foundation for the development of novel therapeutic modalities to treat platelet-associated clinical disorders associated with bleeding and clotting, as well as strategies aimed at improving how platelets are stored and transfused. RELEVANCE (See instructions): Platelets circulate through the bloodstream, serving as cellular sentries to control bleeding at sites of vascular injury. Dysregulated platelet function, however, leads to acute myocardial infarction and stroke. In addition, the ability to collect, store, and transfuse functional platelets capable of restoring hemostasis to thrombocytopenic patients continues to be a challenging problem in the field of transfusion medicine. This proposal examines the properties of platelets that regulate their ability to participate in these processes.